This invention relates to solid instant set microcellular polyurethanes, and more particularly to such polyurethanes having flame retardant properties. Polyurethanes which are flame retardant are desired, and in some cases required, for many applications, especially as legislation increases in requiring high flame retardancy standards. Fire retardancy of polyurethanes is accomplished by either incorporating the flame retardant into the reaction medium as a separate stream or preblending it with the other reactants. There are basically two practical methods of incorporating fire retardants in polyurethane compositions, namely the additive and the reactive methods. In the additive method, it is well known to add phosphorus containing nonreactive plasticizer compounds to improve flame retardant properties. Examples of such products include ANTIBLAZE 19, a cyclic phosphonate ester, disclosed in Mobil Chemical Company U.S. Pat. Nos. 3,789,091 and 3,849,368. ANTIBLAZE 78, a chlorinated phosphonate ester, and ANTIBLAZE 80, a chlorinated phosphate ester, are other useful flame retardants sold by Mobil Chemical Company. THERMOLIN 101 sold by Olin Chemical Company and FYROL 2xC20 sold by Stauffer Chemical Company are additional use chlorinated phosphate ester flame retardants. The latter product is described in Monsanto Company U.S. Pat. No. 3,192,242. It is also known to use solid additives to achieve fire retardancy in polyurethanes, such as by adding a dispersion grade polyvinyl chloride, antimony trioxide and zinc oxide to the reactants, which is described in U.S. Pat. No. 3,574,149. Also, alumina trihydrate is a conventional inert filler used to achieve fire retardancy in urethanes. These approaches have proven to be inefficient due to the incompatibility of the solid additives with the polyurethane system.
A disadvantage of flame retardant polyurethane compositions using additive type flame retardants is the phenomenon of dripping of the melted polyurethane polymer while it is burning during and shortly after ignition. The flaming droplets may spread the flame beyond the site of initial ignition. Indeed, in some compositions, the formulation would be self-extinguishing except for the fact that the dripping away of the melted polymer constantly exposes a fresh new surface of polymer to the fire.
In the reactive method, it is known to use polybrominated polyols to impart flame retardancy to the polyurethane chains when copolymerized with the other reactants which form the urethane linkage. The most efficient of the reactive flame retardants is dibromoneopentyl glycol (DBNG), which reacts with a polyisocyanate to form a diurethane and has the potential of ring closure upon evolution of HBr. Incorporating the dibromoneopentyl glycol in the reaction medium so that it can be polymerized with the other reactants proves to be a problem in commercial-scale operations, since pure DBNG is a solid having a melting point of 110.degree. C. A commercially available dibromoneopentyl glycol is FR 1138, sold by Dow Chemical Company, which consists primarily of 85% dibromoneopentyl glycol, 10% tribromoneopentyl alcohol and 5% monobromoneopentyl triol and has a melting range of 90.degree.-100.degree. C. Although dibromoneopentyl glycol has been recognized as an excellent flame retardant for unsaturated polyester resins for many years, the product had only limited success in polyurethane applications due to the problems associated with handling and processing this material.
The process involved in burning of a polymer are heating, decomposition, ignition, combustion and propagation. Flameretarding mechanisms interrupt the occurrence of one or more of these processes and are provided by certain elements such as halogens (specifically chlorine and bromine), phosphorus, antimony, boron and arsenic. Halogenated compounds can generate species that interfere with free-radical reactions in the gas phase, thereby hindering combustion. Flame retardants containing halogens are thought to function in the vapor phase as either a diluent and heat sink or as a free radical trap that stops or slows flame propagation. Volatile components are generated from halogenated compounds, and these compounds react with radicals in the gas-phase combustion process. Because HCl and HBr constantly volatilize from the surface, less oxygen diffuses to the surface to react in the thermo-oxidative process; the result is exclusion of oxygen at the substrate surface, elimination of hydrogen halides, and char promotion. Organophosphorus compounds function in the solid phase by forming a char over the substrate, reducing flammability of the polymer by preventing heat and mass transfer necessary for combustion. Phosphorus content is not the only criterion for performance as flame retardant; the ester structure is also important. However, the char forming mechanism has been generally found inefficient by using phosphate ester plasticizers in microcellular polyurethane compositions for passing testing criteria classed V-0 according to the Underwriters' Laboratories 94 Standard Test for Flammability of Plastic Materials for Parts in Devices and Appliances, Jan. 24, 1980, pages 7010, Section 3: Vertical Burn Test for Classifying Material 94 V-0, 94 V-1, or 94 V-2.
Rapid-setting, non-elastomeric, solid urethane compositions have been taught in Newton U.S. Pat. No. 3,378,511, Jones et al U.S. Pat. No. 3,726,827 and Olstowski U.S. Pat. No. 4,000,105, all assigned to the Dow Chemical Company. Newton discloses a composition comprising the reaction product of a polyisocyanate with a polyether polyol in admixture with a modifying liquid ester of a carboxylic acid or its anhydrides having a boiling point above about 150.degree. C. and an organotin compound or an amine compound to provide catalysis. The compositions do not include a flame retardant. Jones et al disclose an improvement over the prior art by presenting improved physical properties at the same or lower concentrations of the liquid modifier. The compositions comprise a liquid prepolymer such as one prepared by reacting an organic diisocyanate with a polyhydroxyl-containing compound which, in turn, is reacted with a liquid polyether polyol containing 3 to 8 hydroxyl groups in conjunction with a non-amine-containing catalyst such as stannous octoate. The compositions do not include a flame retardant. Olstowski discloses rapid-setting, non-elastomeric, solid urethane compositions without the use of a catalyst comprising an amine initiated polyether polyol such as triethanolamine, an organic aromatic polyisocyanate such as polymethylene polyphenylisocyanate and a liquid modifier compound having a boiling point above about 150.degree. C. such as dioctylphthalate. The organic aromatic polyisocyanates can include isocyanate terminated prepolymers prepared by the reaction of amine initiated polyol with an isocyanate. Olstowski describes organophosphorus compounds with a boiling point above about 150.degree. C. as suitable liquid modifiers but not as flame retardants. Fire retardancy properties are not inherent in these compositions.
Many different methods have been taught in the prior art for flame retarding polyurethanes by the incorporation of dibromoneopentyl glycol with and without an organophosphorus compound into the polymer matrix. Because of the reactive hydroxyl groups of DBNG it becomes permanently bound in the polyurethane polymer. Miller et al U.S. Pat. No. 4,194,068 discloses a liquid frame retardant for polyurethane compositions, which contain a mixture of a bromohydrin of pentaerithritol, like dibromoneopentyl glycol, a polyhydric alcohol, like a glycerol initiated polyether polyol with an average molecular weight of about 300, and a sufficient amount of a flame retardant organophosphorus compound to render the mixture a substantially solidsfree liquid at 25.degree. C., one atmosphere pressure. The mixtures are suitable for cellular foams and for rigid cellular foams, which are produced in the presence of blowing agents, like fluorocarbons and/or carbon dioxide generated by the chemical reaction between water and diisocyanate. Wegner U.S. Pat. Nos. 4,275,171 and 4,363,882 disclose blends of dibromoneopentyl glycol and flame retandant plasticizers, like halogenated organophosphorus compounds, suitable to flame-retard flexible polyurethane foams in the oneshot foaming process. These blends of DBNG and phosphorus containing plasticizers in flexible foam do not result in satisfactory non-dripping or char formation characteristics.
in many plastics and polymer systems drying agents are required to prevent reaction with water and subsequent gassing during storage or viscosity buildup in storage containers. Molecular sieves are effective adsorbents made from natural or synthetic crystalline alkali-metal alumino-silicates of the zeolite type. The molecular sieve powder, sold by the Linde Division of Union Carbide Corporation, is added to two-component urethanes to the prepolymer side and/or polyol portion of the formulation prior to reaction. Water is adsorbed preferentially over all other compounds, illustrating the strong advantage of the sieve molecule in scavenging water in polyurethane systems. The water, once adsorbed within the crystalline pore system, is retained there and isolated from the resin formulation, thereby providing resin dryness. The powder is essentially inert to the other formulation components. Such molecular sieves have been known for use to provide moisture-adsorbing characteristics, and one disclosed in Wesch et al U.S. Pat. No. 4,123,587 to assist in flame retardancy, but not in a polyurethane system. Furthermore, neither Wesch et al nor any other known prior art disclose the use of molecular sieves for char forming characteristics.